The present invention relates to a process for the manufacture of salts of poly-alpha-hydroxyacrylic acids which can be used as sequestering agents for metal ions and as builders in detergents, as described in Belgian Pat. No. 776,705 of Dec. 15, 1971 in the name of Solvay & Cie. which is hereby incorporated by reference.
The known methods for the manufacture of these products are generally based on the preparation of an intermediate which is the polymer derived from the corresponding alpha-chloroacrylic acid. They comprise a step involving the hydrolysis of this polymer which, whether or not it has been isolated from the formation medium, is in the form of a gel which is insoluble in water. This polymer is the polylactone derived from the poly-alpha-hydroxyacrylic acid.
In the past, this polylactone has been treated with an aqueous solution of a hydroxide to give the desired salt in the form of a solution in the reaction medium. The poly-alpha-hydroxyacrylic thus prepared is then isolated from its aqueous solution, for example, by evaporation of the water or by precipitation by means of methanol, such as described in Belgian Pat. No. 786,464 of July 19, 1972 in the name of Solvay & Cie.
These processes for isolating the poly-alpha-hydroxyacrylate from its aqueous solution are all difficult to carry out. Thus, the removal of water by evaporation involves the use of complicated equipment and, moreover, this operation requires a great deal of heat because the amounts of water to be removed are very large. The precipitation of the polymer from its solution by means of a non-solvent such as methanol also presents numerous difficulties. For example, when precipitating from a very concentrated polymer solution, the solution of polymer sets solid to give a kind of gum, or when precipitating from a slightly concentrated solution of polymer, a very fine powder is obtained which is very difficult to filter off. It is thus necessary to make a prior readjustment of the concentration of the aqueous solution by dilution or by reconcentration, depending on the particular cases. Moreover, the amounts of non-solvent to be employed are very large.